This invention relates generally to refrigeration and more particularly the invention relates to microminiature refrigerators and method of making same.
Certain materials, called superconductors, have the ability to pass electric current without resistance. Since superconductivity is observed only at temperatures close to absolute zero, one of the main obstacles to extensive use of superconducting devices is the need for reliable, continuous refrigeration. Superconducting devices, such as supersensitive magnetometers, voltmeters, ammeters, voltage standards, current comparators, etc., require a cryogenic environment to operate. Traditionally this has been provided by a bath of liquid helium. The helium is liquified elsewhere and transported to, and transferred to the device Dewar. The labor and complexity of such an operation has severely limited the use of these devices. Many of the above superconducting devices dissipate only a few microwatts in operation while the available cryogenic systems provide a refrigeration capacity of watts, thus the devices are poorly matched to the refrigeration.
In addition, many devices such as optical microscope stages, x-ray diffraction sample holders, electron microscope cold stages, devices for cryosurgery in the brain, for ECG, MCG and EKG measurements, and low noise amplifiers require or benefit from subambient operating temperatures.
Additionally, there are a number of high speed, high power devices such as VLSI (very large scale integration) chips and transmitters that are small, on the order of a centimeter square, and dissipate large amounts of heat, on the order of 10 to 50 watts. Traditional cooling devices, such as fans for convection cooling, are not capable of dissipating this amount of heat without significant increases in temperature above ambient.
Miniature closed cycle refrigerators such as those based on the Gifford-McMahon cycle, Vuilleumier, Stirling, etc., have been developed. These refrigerators, with capacities in the range of 0.5-10 watts, are convenient and compact but, because of their moving parts, they introduce a large amount of vibration and magnetic noise which interferes with the operation of the devices. Miniature Joule-Thomson refrigeration systems have been developed which have a cooling capacity typically between 0.5-10 watts. The design configurations of these compact systems are generally helically finned tubes coiled around a mandrel, the high-pressure gas flowing inside the tubes and the low-pressure gas flowing outside the tubes. Such helically finned and coiled heat exchangers are fabricated by laborious welding or soldering of the individual components. Because of the intricacy of the device, microminiature refrigerators with milliwatt capacities until now have not been made practically available.
What is needed for many devices is a microminiature refrigerator of approximately 1/2" to 4" in size with a cooling capacity in the milliwatt range. Also needed are microminiature refrigerator fabrication methods which avoid conventional laborious welding or soldering techniques and allow the formation of very small gas lines to operate the heat exchangers in the laminar flow regime and still have an efficient exchange of heat. The consequent absence of turbulence in the gas stream eliminates vibration and noise, both important considerations for superconducting device applications. The miniature size would allow the incorporation of an entire cryogenic system-superconducting sensor as a hybrid component in electronic circuitry. The microminiature refrigeration capacity would allow the matching of the refrigeration system to the load. The invention meets these needs.
Also needed are microminiature refrigerators of the same general dimensions as discussed above that can dissipate large amounts of heat, 10-50 watts, generated by certain small devices while maintaining ambient or subambient operating temperatures. And such refrigerators should be easy to manufacture and in configurations that are compatible with standard electronic packaging.
A microminiature refrigerator requires scaling down a conventional refrigerator by a factor of about a thousand. The design parameters for a microminiature refrigerator of the same efficiency as a conventional refrigerator using turbulent flow are described in "Scaling of Miniature Cryocooler to Microminiature Size," by W. A. Little, published in NBS Special Publication in April, 1978, which is hereby incorporated by reference.
In summary, the diameter d of the heat exchanger tubing, 1 the length of the exchanger and t the cooldown time are related to the capacity which is proportional to m the mass flow, in the following manner: EQU d.apprxeq.(m).sup.0.5 EQU l.apprxeq.(m).sup.0.6 EQU t.apprxeq.(m).sup.0.6
A microminiature turbulent flow refrigerator with a capacity of a few milliwatts should have d=25.mu. and l a few centimeters.
As the device becomes smaller and smaller, eventually the mass flow becomes too small to allow turbulent flow of the fluid to occur. Laminar flow operation then becomes possible without loss of refrigeration efficiency and gives improved performance.
The theoretical basis for designing microminiature refrigerators using laminar flow heat exchangers is discussed in "Design Considerations for Microminiature Refrigerators Using Laminar Flow Heat Exchangers," presented by W. A. Little at the Conference on Refrigeration for Cryogenic Sensors and Electronic Systems, Boulder, Colo., Oct. 6 and 7, 1980, which is hereby incorporated by reference.
For microminiature heat exchangers operating in the laminar flow region over the same pressure regime and having the same efficiency, the length of the exchanger (l) should be made proportional to the square of the diameter (d) of the exchanger tubing. For example, an Joule-Thomson exchanger operating with N.sub.2 at 120 atmospheres, 5 cm long with fluid flow passages 110 microns wide and 6 microns deep should provide approximately 25 milliwatts cooling. Different refrigeration capacities can be obtained by varying the width of the channel with no change of the efficiency. One may thus operate under stream-lined conditions free of vibration and turbulence noise, an advantage, particularly for superconducting devices, which require a very low noise environment.
In a Joule-Thomson refrigerator of this type it is normally convenient to use a capillary channel to throttle the compressed gas; however, it is common knowledge that a porous structure such as porous metal, sintered ceramic, etc. can be used equally well for throttling the gas.
In order to construct microminiature refrigerators, new fabrication techniques are needed for producing heat exchangers and expansion nozzles, a factor of 100 to 1,000 times smaller than those of conventional refrigerators.
Conventional fabrication techniques are ill-suited for microminiaturization since channels of the order to 5-500 microns must be formed accurately and the device must be sealed so as to withstand high pressure of the order of 150-3000 psi for refrigeration efficiency.
Accordingly, an object of the present invention is a novel refrigerator.
Another object of the present invention is a novel microminiature refrigerator with a cooling capacity ranging from milliwatts up to 50 watts or more.
Another object of the invention is a novel single-stage cryogenic microminiature refrigerator.
Another object of the invention is a novel multistage cryogenic microminiature refrigerator.
Another object of the invention is a novel method of manufacturing a microminiature refrigerator.
Yet another object of the invention is a novel method of making a microminiature refrigerator using photolithographic and chemical etching techniques.
Still another object of the invention is a novel method of making a microminiature refrigerator using a fine-particle sandblasting technique.
A further object of the invention is to provide a novel refrigerator of small size comprising two or more plates of a low thermal conductivity material such as glass bonded pressure-tight and containing at one or more plate interfaces micro-sized gas supply and return passages to a chamber that is adapted to continuously cool a superconductor or like device. Pursuant to this object the inlet gas pressures may be in the order of 150-3000 pounds per square inch, and the passages may be in the range of 5-500 microns wide and 5-60 microns deep.
Pursuant to the foregoing object of the passages and cooling chamber may be formed by recessing plate surface areas or forming raised channel walls at the interface or interfaces of the plates.
And still another object of the invention is a novel method of making a microminiature refrigerator by forming raised channel walls.